Re: [PATCH v5.4.y, v4.19.y] mm: memcg/slab: fix memory leak at non-root kmem_cache destroy

[Roman Gushchin <guro@fb.com>]

On Wed, Jul 15, 2020 at 01:32:00PM +0200, Vlastimil Babka wrote:
> On 7/7/20 8:27 AM, Muchun Song wrote:
> > If the kmem_cache refcount is greater than one, we should not
> > mark the root kmem_cache as dying. If we mark the root kmem_cache
> > dying incorrectly, the non-root kmem_cache can never be destroyed.
> > It resulted in memory leak when memcg was destroyed. We can use the
> > following steps to reproduce.
> > 
> >   1) Use kmem_cache_create() to create a new kmem_cache named A.
> >   2) Coincidentally, the kmem_cache A is an alias for kmem_cache B,
> >      so the refcount of B is just increased.
> >   3) Use kmem_cache_destroy() to destroy the kmem_cache A, just
> >      decrease the B's refcount but mark the B as dying.
> >   4) Create a new memory cgroup and alloc memory from the kmem_cache
> >      A. It leads to create a non-root kmem_cache for allocating.
> >   5) When destroy the memory cgroup created in the step 4), the
> >      non-root kmem_cache can never be destroyed.

Hello, Muchun!

If the scenario above is accurate, it means that somebody is allocating
from the kmem_cache A (or it's memcg counterparts, doesn't matter) after
calling kmem_cache_destroy()? If so, it's an API violation, and the following
memory leak is a non-issue on the slab side. No one should allocate memory
after calling kmem_cache_destroy(). It has to be called after all outstanding
allocations are freed, and it should be literally the last operation
with the kmem_cache.

Kmem_cache aliasing/sharing, as well as memcg accounting implementation are
implementation details and should not affect the picture.

I wonder, did you see the problem in the wild? How does it look like?
Which kmem_cache is involved? Etc.

BTW, Vlastimil is absolutely right about stable backports and rework planned
for 5.9, but let's figure out the problem first.

Thank you!

> > 
> > If we repeat steps 4) and 5), this will cause a lot of memory leak.
> > So only when refcount reach zero, we mark the root kmem_cache as dying.
> > 
> > Fixes: 92ee383f6daa ("mm: fix race between kmem_cache destroy, create and deactivate")
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> 
> CC Roman, who worked in this area recently.
> 
> Also why is this marked "[PATCH v5.4.y, v4.19.y]"? Has it been fixed otherwise
> in 5.5+ ?
> 
> > ---
> >  mm/slab_common.c | 43 +++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 41 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 8c1ffbf7de45..83ee6211aec7 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -258,6 +258,11 @@ static void memcg_unlink_cache(struct kmem_cache *s)
> >  		list_del(&s->memcg_params.kmem_caches_node);
> >  	}
> >  }
> > +
> > +static inline bool memcg_kmem_cache_dying(struct kmem_cache *s)
> > +{
> > +	return is_root_cache(s) && s->memcg_params.dying;
> > +}
> >  #else
> >  static inline int init_memcg_params(struct kmem_cache *s,
> >  				    struct kmem_cache *root_cache)
> > @@ -272,6 +277,11 @@ static inline void destroy_memcg_params(struct kmem_cache *s)
> >  static inline void memcg_unlink_cache(struct kmem_cache *s)
> >  {
> >  }
> > +
> > +static inline bool memcg_kmem_cache_dying(struct kmem_cache *s)
> > +{
> > +	return false;
> > +}
> >  #endif /* CONFIG_MEMCG_KMEM */
> >  
> >  /*
> > @@ -326,6 +336,13 @@ int slab_unmergeable(struct kmem_cache *s)
> >  	if (s->refcount < 0)
> >  		return 1;
> >  
> > +	/*
> > +	 * If the kmem_cache is dying. We should also skip this
> > +	 * kmem_cache.
> > +	 */
> > +	if (memcg_kmem_cache_dying(s))
> > +		return 1;
> > +
> >  	return 0;
> >  }
> >  
> > @@ -944,8 +961,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
> >  	if (unlikely(!s))
> >  		return;
> >  
> > -	flush_memcg_workqueue(s);
> > -
> >  	get_online_cpus();
> >  	get_online_mems();
> >  
> > @@ -955,6 +970,30 @@ void kmem_cache_destroy(struct kmem_cache *s)
> >  	if (s->refcount)
> >  		goto out_unlock;
> >  
> > +#ifdef CONFIG_MEMCG_KMEM
> > +	mutex_unlock(&slab_mutex);
> > +
> > +	put_online_mems();
> > +	put_online_cpus();
> > +
> > +	flush_memcg_workqueue(s);
> > +
> > +	get_online_cpus();
> > +	get_online_mems();
> > +
> > +	mutex_lock(&slab_mutex);
> > +
> > +	if (WARN(s->refcount,
> > +		 "kmem_cache_destroy %s: Slab cache is still referenced\n",
> > +		 s->name)) {
> > +		/*
> > +		 * Reset the dying flag setted by flush_memcg_workqueue().
> > +		 */
> > +		s->memcg_params.dying = false;
> > +		goto out_unlock;
> > +	}
> > +#endif
> > +
> >  	err = shutdown_memcg_caches(s);
> >  	if (!err)
> >  		err = shutdown_cache(s);
> > 
>